1. Field of the Invention
The present invention relates to an x-ray mask for a fine pattern-forming technique, an x-ray exposure device, an x-ray exposure method, and a semiconductor device manufactured though the x-ray exposure method. More particularly, the present invention relates to a technique allowing for a fast and fine pattern transfer as compared with the conventional technique, for use in a system of transferring a fine pattern formed on an x-ray mask by an x-ray proximity exposure technique, in a technique of transferring a fine pattern mainly for manufacturing a semiconductor integrated circuit.
2. Description of the Background Art
A conventional x-ray proximity exposure method is schematically shown in FIG. 9. X-rays 1 emitted from an electron storage ring 10 are condensed by a mirror 2 into a prescribed range. X-rays 1 transmitted through a vacuum protective filter 3 radiate onto a wafer 6 with a resist 5 arranged in the proximity of an x-ray mask 4 at a prescribed distance. X-ray mask 4 is formed of an x-ray transmission body 40a and an x-ray absorber 40b with a pattern formed thereon, and x-rays 1 transmitted through x-ray transmission body 40a and x-ray absorber 40b radiate onto resist 5 on wafer 6. Resist 5 that absorbs x-rays 1 produces secondary electrons, which cause a chemical reaction of resist 5. The x-ray mask pattern is thus transferred to resist 5.
Resolution of x-ray proximity exposure is said to be determined by two factors: (i) resolution limit of an aerial image determined by Fresnel diffraction and (ii) resolution limit determined by so-called secondary electron scattering, that is, photoelectrons and Auger electrons produced in the resist by radiation of exposure light scatter into an infinite region.
The secondary electron scattering in (ii) increases with higher electron energy and reduces with lower electron energy. Photoelectrons of secondary electrons have higher energy and lower the resolution as the absorption wavelength of x-rays becomes shorter. On the other hand, in (i), the image of x-ray absorption energy formed in a resist is dependent on Fresnel diffraction of x-rays transmitted through the x-ray mask, and resolution limit R can be expressed by the following equation (1):R=k(λ·G)1/2  (1)where k is a constant depending on the mask absorber material, the mask pattern geometry and the exposure system. λ represents the wavelength of x-rays absorbed in the resist, and G represents the distance between the mask and the wafer (exposure gap). From the above equation (1), it can be understood that the shorter the x-ray wavelength is or the narrower the distance between the mask and the wafer is, the higher the resolution becomes, and the longer the x-ray wavelength is or the wider the distance between the mask and the wafer is, the lower the resolution becomes. However, since the resolution is worsened due to photoelectrons scattering in the resist with shorter wavelength, in practice, a wavelength having such a range is used in that the photoelectrons scattering is not increased as compared with a pattern size.
The following three methods have been proposed as a method of transferring a pattern with a small k. A first method is described in J. Vac. Sci. Technol., B16(1998), p. 3504 in which an isolated opening pattern is formed by optimizing a mask pattern geometry. A second method is described in Jpn. J. Appl. Phys., Vol. 38(1999), p. 7076 in which a periodic line-and-space pattern is formed by optimizing a contrast and a phase shift amount of an x-ray absorber. A third method is described in J. Vac. Sci. Technol., B19 (2001), p. 2428 in which a pattern is formed by performing multiple exposures. Any of the methods allows for formation of a finer pattern without changing exposure gap or wavelength.
First, the problem of the above first exposure method described in J. Vac. Sci. Technol., B16 (1998), p.3504 to obtain an isolated opening pattern will be described. FIG. 10 is a schematic diagram illustrating a Fresnel annular zone. The light intensity at a position P on the resist surface is determined by contribution of all light transmitted through the mask and reaching position P. When the mask surface is divided into regions expressed by so-called Fresnel annular zones, Fresnel diffraction is characterized in that contributions to the intensity of light transmittance through the mask surface onto position P on the resist surface are cancelled between the adjacent Fresnel annular zones because there is a difference of π radians in the phase of x-rays reaching position P.
Here the n-th Fresnel annular zone refers to an annular region between radius (G·λ·(n−1))1/2 and (G·λ·n)1/2 with respect to a mask position Q as the center vertically above position P. In order to realize an ideal state in which the intensity is maximized at position P, the phase in an even-numbered Fresnel annular zone may be shifted by π radians for attaining the same phase at position P.
In the exposure method described in J. Vac. Sci. Technol., B16(1998), p.3504 to form a fine isolated pattern, light contributing to the same phase is taken in as much as possible by making the size of an opening equal to the size of the first Fresnel's annular zone and by shielding the other region with an x-ray absorber, and light rays opposite in phase that cancel each other are reduced by the x-ray absorber. Accordingly, the intensity on the resist at the center of the opening is increased so that the resolution is improved.
Since the annular region is varied with the exposure gap and the wavelength, appropriate exposure gap and wavelength have to be selected in accordance with a pattern. In this method, the phase difference by the light transmitted through the x-ray absorber positioned in a second Fresnel annular zone is not π radians that is the opposite phase, but (1/2)·π radians in total, since the phase shift amount of the x-ray absorber is produced by (−1/2)·π radians. Therefore the cancellation effect of the second Fresnel annular zone is relieved and the intensity as position P is effectively increased.
Unfortunately, however, the phase in the second Fresnel annular zone is smaller by (1/2)·π radian as compared with π radians as an ideal state of the phase shift amount and the intensity increasing effect at position P is suppressed. Furthermore, provided that the thickness of the x-ray absorber is increased in order to bring the phase shift amount of the x-ray absorber close to πradians, the processing becomes difficult. In addition, since the transmittance of the x-ray absorber is reduced, the contribution to the intensity at position P is undesirably reduced.
Next, the problem of the above second exposure method described in Jpn. J. Appl. Phys., Vol. 38(1999), p.7076 to form a fine line-and-space pattern will be described.
In this method, the phase shift amount of the x-ray absorber is (−1/2)·π radian, and an optical path difference of light transmitted through the x-ray absorber and light transmitted through the x-ray transmission body is (1/2)·π radian, so that both light rays are of the same phase on the resist immediately below the x-ray transmission body and of opposite phase on the resist immediately below the x-ray absorber. Since the light transmitted through the x-ray absorber is used for pattern formation, this effect is not achieved if the mask contrast of the x-ray absorber (a value of the quantity of light transmitted through the x-ray transmission body divided by the quantity of light transmitted through the x-ray absorber) is high.
On the contrary, if the mask contrast of the x-ray absorber is too low, optical interference takes place and an unnecessary pattern is formed on the resist immediately below the x-ray absorber. Therefore a mask contrast of approximately three is used. Though this attains an aerial image having a relatively high contrast, the period of the transfer pattern cannot be made smaller than the period of the mask pattern.
Next, the problem of the third exposure method described in J. Vac. Sci. Technol., B19(2001), p.2428 to obtain a periodic pattern will be described. In this exposure method, by utilizing the fact that resolution of the resist pattern is high where the pattern has a long repeated period, close to an isolated pattern, a resist pattern having a period less than half the mask pattern period is formed by multiple exposures with displaced exposure positions. In case of the this method, however, the increased number of exposures reduces throughput.